This invention relates to a process for forming a plate-like component, such as a fuel tank bottom segment for a spacecraft.
To produce double-curved components made of a hardened metallic material, it is known to form a plane, pie-shaped blank simultaneous with the thermal hardening, by clamping the blank to a mold shell having a curved contour corresponding to the desired component contour, and subjecting it to externally applied pressure during the thermal hardening cycle. The intensity of the pressure is selected so that, taking into account its thermal hardening temperature, the creep resistance of the material is exceeded, and that the blank conforms to the contour of the mold shell in the course of the artificial aging. However, with this so-called age creep forming, substantial spring-back effects occur after the forming which, particularly in the case of large-surface blanks, cause considerable deviation of the resulting component geometry from the contour of the mold shell. In the case of thin components, there is also the danger of local wrinkling. To prevent such wrinkling, the component must be thickened beyond the structural required thickness, which is a serious disadvantage with respect to material and weight efficiency, particularly of components for aviation and space travel. In addition, the characteristics of the material, such as the mechanical strength and the yield point of the component thus produced are below the maximum values which are achievable for the material.
An alternative production process for components of this type is known as shot peen forming. In this process, a hardened component blank is formed into a double curved contour with precision and relatively low production expenditures and without the above-mentioned production-related difficulties concerning age creep forming. However, this process suffers from the disadvantage that the material characteristics of the finished component, specifically its residual ducticity, do not reach optimum values which can be realized for the selected material as such.
An object of the invention to provide a manufacturing process of the generic type described above, which is simple and can achieve improved mechanical characteristics of the material of the formed and hardened component.
This object is achieved by the process according to the invention, which is based on the principle that the residual ducticity of a shot-peen-formed thermally hardened component is significantly increased, and variations in its material strength are considerably reduced if, in its still unhardened condition, the component is shot-peen-formed and then artificially aged under external pressure which causes creep effects in the material. As a result of the combination of peen-forming and thermal hardening of the workpiece blank with a simultaneous application of external pressure, a high quality component is obtained with material characteristics that are clearly improved in comparison to each of the initially mentioned production processes. Even large surface, double-curved components made of metallic, thermally hardening materials (such as nonferrous metal alloys, as they are used in aviation and space travel) can be produced, while the material is utilized in a weight-saving, precisely contoured and simple manner, at reasonable cost.
In a particularly preferred embodiment, during the artificial aging the thermal hardening temperature is maintained up to approximately 12% and the thermal hardening time is maintained to approximately 20% below the values for the unformed material, as specified in commonly available material specification pamphlets. Because of the above-mentioned shot-peen-forming, the maximum hardening effect of the artificial aging shifts in the direction of lower thermal hardening temperatures and times, and as a result, the mechanical component characteristics and particularly the material strength are further improved.
In another preferred embodiment, the shot-peen-formed component is placed under pressure during the artificial aging process in a full-surface mold shell, with a mold face corresponding to the final component contour. In this manner, during the creep process generated by the pressure and the temperature during the thermal hardening, the formed component is maintained in a stable form, and inaccuracies of the contour, which may remain in the component after the peen-forming and internal tensions resulting in back-springing, are automatically compensated in a simple manner.
In order to ensure a uniform surface pressure in the pressing tool with low expenditures for manufacturing means, even for components which have a smooth curved surface on only one side, but are non-uniformly profiled on the opposite component side (being thickened in an approximately step-shaped manner), the mold space of the pressing tool on the component side facing away from the mold face is expediently sealed off by a flexible boundary wall which is pressured by an external pressure medium during the artificial aging. The action by the pressure medium and the artificial aging preferably take place in an autoclave, and the mold space between the flexible boundary wall and the mold face is evacuated to increase component compression.